A method for accurately measuring the amount of each of two differently radiolabled chemical species in a mixture has been developed. This method has been utilized, in conjunction with an automated microfractionator previously developed in this laboratory, to quantitate the concentration gradients, formed under the influence of centrifugal force fields, of each of two solute species in a mixture. A theory of sedimentation equilibrium has been extended to solutions of one or two solute components, which may undergo self- or hetero-association, at arbitrarily high concentrations. Simple yet accurate semi-empirical algorithms have been developed to calculate the true weight-average molecular weight of each component from the experimentally measured apparent weight-average molecular weight. The kinetics of formation of concentration gradients in a solution subjected to a time-varying centrifugal force are being studied by means of computer simulation.